Abstract:Based on GPCC_V7 (Global Precipitation Climatology Centre Version 7) and ERA-20C (European Centre for Medium-Range Weather Forecasts atmospheric reanalysis of the 20th century) monthly precipitation data, multi-decadal variability (MDV) of precipitation in arid and semi-arid region of northern China and the relationship between MDV and Pacific Decade Oscillation (PDO) are analyzed using ensemble empirical mode decomposition (EEMD) and composition analysis methods. The result shows that precipitation in arid and semi-arid region of northern China has a multi-decadal variability with an average period of about 50-60 years, which may be modulated by PDO. Precipitation in northern Xinjiang and the Inner Mongolia are significantly positively correlated with PDO, while that in eastern and western Ordos Loop region is significantly negatively correlated with PDO. Further study shows that meridional circulation intensifies during positive PDO phase, leading to more southward water vapor transport from the Arctic Ocean. As a result, precipitation in Xinjiang increases due to the convergence of cold, moist northwesterly flow from the Arctic Ocean and warm, moust southwesterly flow from Arabian Sea in southern Xinjiang. On the other hand, convective heating over the equatorial western Pacific Ocean and Indian Ocean leads to the Pacific Japan/East Asia-Pacific(PJ/EAP) pattern of teleconnection. It is favorable for the transport of moist airmass from Bohai Sea to its northern area and increases precipitation there during positive PDO phrase. Besides, the moist northerly flow from Bohai and westerly flow converges in the northern Ordos Loop area, where a maximum precipitation center forms. The situation is opposite during negative PDO phrase.

Abstract:This study investigates the distribution of the North Atlantic Storm Track (NAST) in the upper and lower troposphere, and associated eddy kinetic energy (EKE) distribution in each layer. EKE is then isolated in each layer. The results shows that there is a great difference in NAST and EKE between the upper and lower troposphere. In the upper troposphere, the NAST stays a bit further southwest; in the lower troposphere, the NAST shifts poleward. By examining each term in the EKE equation, it is found that the barotropic conversion is opposite in the two layers, which could be responsible for the great difference in their storm track distribution patterns. That is to say, the barotropic conversion mainly damps EKE in the upper layer whereas it strengthens EKE in the lower layer because the barotropic conversion is negative in the upper layer but positive and shows the maximum value in the center of NAST in the lower layer. Furthermore, the baroclinic conversion and the ageostrophic geopotential flux divergence both contribute to the maintenance of EKE in the upper layer.

Abstract:Discharge fluxes into the ocean are an important component of material and energy cycle in estuarine and coastal areas. They are also the primary controlling factor of morphological and environmental evolution. Based on statistical analysis of rainfall and runoff of tidal rivers with sluices at Yancheng coast from 1957 to 2012, the authors studied features of multi-scale changes in runoff affected by tidal sluices using M-K trend test and wavelet analysis, etc. Results of the present study have great implications for estuarine and coastal management under the influence of climate change and sea level rise. It is found that runoff into the ocean decreases at a rate of 3×10⁶ m³ (10 a)^-1, seasonal and decadal variability in runoff are obvious, and statistical characteristics of runoff differ greatly before and after 1965. Runoff variation displays a multi-scale feature, and the primary period is 23.38 a. Runoff and rainfall demonstrate alternating "high-low" and "wet-dry" conditions respectively at different scales. In addition, ENSO events and sluices at estuaries could affect the inter-annual, inter-decadal and seasonal variability of runoff. Runoff estimation and evaluation of runoff changes should be conducted in the future with consideration of climate and land surface changes.

Abstract:In this paper, the authors discussed a record heavy snowfall event that occurred in North China in November of 2009. The conventional and unconventional data including satellite images and Doppler radar and automatic weather station (AWS) observations were utilized. The related physical quantities including the frontogenesis function, the kinetic energy budget and moisture flux were computed and the formation mechanism for the heavy snowfall was investigated. Major conclusions are as follows:(1) The returning flow accompanied with disturbances intensified the moisture transport and horizontal convergence in North China. The "inverted trough" to the south of the cold high pressure in the lower troposphere also played an important role in the formation of the heavy snowfall. (2) Radar data revealed clearly that a convergence zone maintained stably to the southwest of Shijiazhuang City, where convective cloud clusters developed continuously and directly led to the formation of heavy snowfall. (3) In the T-lnp diagrams of sounding data at Taiyuan and Xingtai, the frontal zone with the inversion layer could be seen clearly, which was favorable for the accumulation of unstable energy. However, the "warm nose" did not exist, which explains why only the snowfall occurred and there existed no freezing rain. (4) Based on the above discussions, a conceptual model for North China heavy snowfall was proposed.

Abstract:Based on an air pollution index, conventional pollution observations, surface meteorological observations and satellite remote sensing data, the relationships between air pollution, large-scale circumfluence and boundary layer meteorology associated with this event were comprehensively analyzed. Additionally, characteristics of wet removal during precipitation were investigated by Laser Precipitation Monitor data. The results showed that:1) Stable atmospheric stratification and weak winds caused by the steady and uniform high-pressure field contributed to the occurrence, development, maintainability, and reinforcement of the pollution event. The remote transport of pollutants had a significant impact on ambient air quality of Wuhan Area; 2) Wet removal of aerosols occurred during the precipitation event. For example, the washout rate of particle pollutants was the highest, following by that of NO₂, SO₂, and CO, and the washout rate of O₃ was the smallest. By comparing the parameterization of the washout rate with the average scavenging coefficient E (Scott, 1982) and that of Wuhan, it was found the E value was around 0.25 to 0.35 in Wuhan area.

Abstract:Urban heat island (UHI) is a very important local climate phenomenon. With the rapid development of global warming and urbanization, UHI effect has received more attention and research. Previous studies have shown that population aggregation and population size have a significant impact on UHI effect and regional urban climate. However, whether and to what extent the massive human migration can affect the urban climate remain unclear. The world's largest annual human migration, i.e. the population movement around the Chinese New Year (CNY), is taken as an example in the present study to investigate the effects of massive human migration on UHI during the CNY holiday for the period of 2000-2015 in Shanghai. It was found that the daily maximum (△UHⅡ_max), mean (△UHⅡ_mean), and minimum (△UHⅡ_min) surface temperature difference between urban and nearby rural stations averaged over the period 2000-2015 during the CNY week were 0.35℃, 0.90℃, and 1.53℃, respectively, which were 0.16℃ (31.4%), 0.11℃ (10.9%), and 0.15℃ (8.9%) lower than those during the background period (2-4 weeks before and 2-4 weeks after the CNY week), respectively. Our findings highlight the important role of massive human migration on urban climate based on a case study in Shanghai. Meanwhile, the results also indicate that the influences of human migration on urban climate are complex, which are often intertwined with local background climate, weather condition, city size, and the characteristics and pattern of human migration and even firework prohibition (by means of impacting the aerosols, cloud cover, and waste heat release). Therefore further studies on this issue are necessary.

Abstract:In this study, the last millennium transient simulation of Integrated Climate Model (ICM) climate model developed in Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences is presented. The model was driven by up-to-date external forcings, and the simulation could reproduce many large-scale climate variations of surface temperature and atmospheric oscillation suggested by reconstructions. The centennial scale changes of global monsoon were further studied. Results indicate that the overall variations of temperature and atmospheric oscillation during the last millennium are reasonably reproduced by ICM. The northern hemisphere surface temperature is higher than average during 900-1200 but lower than average during 1500-1800. The simulated hemispheric scale surface temperature shows an unprecedented warming from the 1800s to present. The simulation didn't well reproduce the reconstructed Atlantic Multi-decadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO). Over the past millennium, the simulated global monsoon shows significant centennial variations. The global monsoon strengthens during 850-1050, 1150-1200, and 1300-1420. Conversely, it weakens during 1210-1300 and 16000-1850. In the last century, the global monsoon index shows a prominent upward trend. During the Medieval Warm Period (MWP), precipitation increases in most parts of the global monsoon regions, but decreases within the Little Ice Age (LIA). The global monsoon strengthens markedly during the Present Warm Period (PWP), and the precipitation rate in the equatorial western Pacific Ocean increases by 1mm/d.

Abstract:Land Use/Cover Change (LUCC) is an influencing factor of regional climate change that cannot be ignored. However, there still exists no highly reliable Land Use/Cover data so far, and thus great uncertainties remain in the study of the effects of LUCC on climate change. For this reason, two datasets from China Statistical Yearbook and observed MODIS (MODerate-resolution Imaging Spectroradiometer) respectively are used to compare and analyze changes in three typical land use/cover types (forest, urban, cropland) over administrative regions. The results show that:(1) Spatial distribution and changes in the forest cover in provinces (municipalities, autonomous regions) of China overall agree well during 2004-2011 between China Statistical Yearbook and MODIS data. Both China Statistical Yearbook and MODIS data show that forest cover increases to different degrees in most provinces (municipalities, autonomous regions) of China but decreases in Beijing, Tianjin, Jilin, Heilongjiang, Shanghai, and Jiangsu based on MODIS data. MODIS data can accurately reflect the total forest cover, but there exist large errors in single forest types and their changes due to low classification accuracy. (2) The area of urban built districts from China Statistical Yearbook and its increment are much higher in eastern China than in the western region. Huanghuaihai region and southeastern coastal region of China show an accelerated expansion trend of urban area, which is consistent with the actual situation. However, MODIS data cannot reflect the process of rapid urbanization development in China over the past decade. The possible reasons are that urban areas are usually small with complex surface. Limited by spatial resolution, the classification accuracy of MODIS land cover is low in discrete regions, leading to no monitoring results of expansion of the new cities. (3) Both the values and spatial distribution are in good consistency among MODIS cropland, sown area of farm crops and irrigated area from China Statistical Yearbook, and the largest coverage of these three data are all in Huanghuaihai region. However, change quantities of the three data show great difference during 2001-2011. In particular, MODIS cropland and irrigated areas from China Statistical Yearbook increase in provinces (municipalities, municipalities) in eastern China, while sown area of farm crops from China Statistical Yearbook decrease. (4) The change rate of urban built districts in China Statistical Yearbook is the largest (6.25%) among all types of land use changes over China during 2004-2011, followed by forest. Because the quantity of arable land is large and the changes account for a relatively small proportion of the arable land, the change rate of MODIS cropland, sown area of farm crops and irrigated area are all small.

Abstract:A comprehensive understanding of heatwaves is of great importance for making scientifically-based emergency plans to reduce their consequent disastrous impacts. Based on daily maximum temperature observations in summer over Hunan Province during 1960-2014, the spatial and temporal distribution characteristics of heatwaves and the possible synoptic causes are discussed. The major conclusions are as follows. High occurrence frequency and intensity of heatwaves are commonly observed in eastern Hunan (near Hengyang Basin), while heat waves are less frequent and with weak intensity in western Hunan. The areas affected by heatwaves are expanding and the intensity of heat waves is also enhancing, whereas the spatial difference in heatwaves has narrowed. The heatwave frequency (intensity) displays obvious stage changes, which decreased (weakened) before 1980s but turned to increase (strengthen) since the 1990s. After entering the 21st century, more frequent and stronger heatwaves take place. The analysis of a heat wave event with extremely high temperature showed that a stable and strong western subtropical high with its position shifting westward and northward, eastward extending South Asian high, and strong and persistent downdrafts are all favorable for the occurrence of high temperature in Hunan.

Abstract:The severe haze occurred over North China and Huanghuai area in 16-21 December 2016 was analyzed and the atmospheric circulation anomalies were diagnosed to reveal one of the important causes. The coverage of this severe haze was 71×10⁴ km², and the duration was six days. The highest measured hourly PM2.5 concentration was above 1100 μg/m³. The East Atlantic/West Russia and West Pacific teleconnections (mid-upper level) and Arctic Oscillation (near surface) were in their positive phases and affected local atmospheric circulations and meteorological conditions, which were favorable for the occurrence of haze. The anti-cyclone over North China and Huanghuai area effectively confined the vertical motion and weakened the surface wind speed. Meanwhile, there existed an obvious weak low pressure system near the surface with warm and humid southerly winds. From the perspective of local meteorological conditions, the weak surface wind and high humidity condition as well as the shallow boundary layer were most important contributors to the occurrence of the severe haze in 16-21 December 2016.